Benefit is claimed of the prior filing date of Mexican application no. PA/a/2001/007229, filed Jul. 13, 2001 in accordance with 37 CFR xc2xa71.55 AND 35 USC xc2xa7119.
This invention relates to a method to produce a cement clinker and, specifically, a method to produce cement clinker using coke with a high-sulfur content as fuel, in order to minimize the problems related to the use of said fuel.
The processes and plants used to manufacture cement clinker are well known. Generally, the manufacturing process consists in preparing a raw meal (raw mixture) comprising a mixture of materials such as limestone (CaCO3), clay (argillaceous) materials (e.g., SiO2, Al2O3) and iron minerals (e.g., Fe2O3). Said raw meal preparation includes the steps of drying, pulverizing and supplying said materials in adequate proportions to achieve a clinker with the required composition to obtain a product having the final quality desired. Once the raw meal is prepared, pulverized and homogenized with the required composition, the next steps in the process are: feeding said raw meal to a kiln passing through a pre-heater; calcining said preheated raw meal to transform the CaCO3 into CaO and C02; feeding the calcined meal to a sintering kiln; sintering (also known as clinkering) the calcined meal to form the clinker compounds such as tricalcium silicate (alite), dicalcium silicate (belite), tricalcium aluminate and tetracalcium aluminoferrite.
Typically, in order to carry out this drying, calcining and sintering process (clinkering), high amounts of energy are required to maintain the high temperatures in the process, the required temperature in the sintering step being about 1450xc2x0 C., which to achieve and maintain requires the consumption of large amounts of fuel.
Currently, the availability of fuels with a high-sulfur content and their lower cost represent an opportunity for the cement industry, however, their use requires special operation and handling with new methods to facilitate their use without detriment to the continuous operation of the kiln.
One of the available lower cost fuels is petroleum coke having a high-sulfur content. Due to its nature, it typically has a high sulfur content of more than 4.5% elemental sulfur. The sulfur included in the fuel has a significant effect when included in the combustion process. On the one hand, this type of fuel generates the heat necessary to maintain the process temperature. On the other hand, it produces SO2 which, added to the SO2 coming from the sulfur content in the raw materials used to form the raw meal, represents a significant amount. This sulfur content can produce blockages in the preheater, if steps or special measures are not taken or adequate compensating methods are not used for processing.
As is known, the SO2 contained both in the combustion gases as well as in the raw meals come in direct contact with the CaO (lime), forming sulfated calcium compounds (calcium sulfites and sulfates CaSO3 and CaSO4). When subjected to the high temperatures for forming the clinker compounds, which are higher than the decomposition temperature, the sulfated compounds decompose again into SO2 and CaO. The latter reacts to form new clinker compounds, and the SO2 carried in the combustion gases returns back towards the kiln solids inlet to again react with incoming fresh CaO. In this way, a continuous cycle is set up with ever increasing sulfur concentrations. Upon reaching high enough concentrations of SO2, and having no outlet, sulfur compounds precipitate on the coldest areas of the preheater forming accumulations and blockages, such as in the preheater, the fume chamber, etc. This causes operational disturbances by reducing efficiency and, in more serious cases, stops the operation altogether; for example, when rings form in the clinkering kiln.
In the art, there have been a number of efforts attempting to design installations, equipment and/or processes to enable the use of high sulfur content solid fuels by seeking to solve the problems related to the formation and excessive accumulation of SO2. However, the majority of the processes and/or plants to manufacture cement clinker using high sulfur content solid fuel present certain disadvantages with respect to the complexity of the processes and equipment, as well as high costs.
Examples of such prior art are to be found in patents discussed below(which are incorporated herein by reference).
See for instance, the U.S. Pat. No. 4,465,460, entitled xe2x80x9cProduction of cement clinkerxe2x80x9d issued to Paul Cosar on Aug. 14, 1984. This relates to the use of high sulfur content solid fuels in the production of cement clinker in rotating kilns. The solid fuel is gasified and the gasified fuel is contacted with a fraction of the raw material used for the production of cement clinker to desulfurize the gasified fuel. The gasified desulfurized fuel is then discharged towards calcining step to provide calories to the same. In addition, it is mentioned that the factors to increase the desulfurization reaction are the gas temperature in the range of approximately 800xc2x0 C. to 950xc2x0 C. and the fine granulation of the raw material, preferably to an average the grain size of approximately 100 microns. This patent requires complex facilities and processes to produce cement clinker, which includes additional equipment to reduce the sulfur content or to desulfurize the combustion gases. The combustion gases are placed in contact with a fraction of the raw material to, thus, desulfurize said combustion gases by the partial reaction of the SO2 with the CaO. However, the control of operation parameters is not mentioned to teach how to avoid the decomposition of the sulfated compounds formed in prior steps, before the calcinated raw material is sintered. In other words, said patent specifically describes a process and installation to reduce to a minimum the sulfated compounds contained in combustion gases specifically. Thus, there is no suggestion as to how effectively to avoid the problems related to the accumulation of sulfated compounds in the overall system.
U.S. Pat. No. 4,662,945 (method) and its divisional U.S. Pat. No. 4,715,811 (apparatus), both entitled xe2x80x9cProcess and apparatus to manufacture low sulfur cement clinkerxe2x80x9d issued to Thomas R. Lawall on May 5, 1987 and Dec. 29, 1987, respectively, describe a process and apparatus to manufacture low sulfur cement clinker from cement raw meal containing sulfur or with fuel containing high levels of sulfur or a combination thereof. The apparatus includes a preheater, calcining kiln, clinkering kiln and clinker cooler. A separate source of fuel is added at the end of the clinkering kiln to maintain reducing conditions and temperatures for decomposing the sulfur compounds. The fuel added at the end of the clinkering kiln is carbon or coke fuel with low volatility. This patent relates to the production of a cement clinker with low sulfur content by means of separating the sulfur compounds under reductive conditions. To obtain said clinkers with a low-sulfur content, the use of added equipment and steps specially designed to operate under said reductive conditions are required. The obtained product is a clinker with a low alkaline sulfur content. Notwithstanding, said patent does not make reference either to the sintering temperature reduction nor to the improvement or pretreatment of the raw material effective to avoid the accumulation of sulfated compounds in the system, but focuses instead on eliminating the sulfated compounds during the calcining step.
The U.S. Pat. No. 6,142,771, entitled xe2x80x9cControl of cement clinker production using high sulfur fuel in a Lelep-Lepol travelling grate rotary kiln by analysis of sulfur in the end productxe2x80x9d issued to Joseph Doumet on Nov. 7, 2000, describes a method and apparatus of producing cement clinker using a high-sulfur content fuel. The high-sulfur content fuel is fed to a burner disposed in the clinkering kiln and mixed with the raw meal before entering the clinkering kiln. The object of this invention is to control the oxygen and carbon monoxide concentrations, as well as the sulfur content in the cement clinker to control the system. To use fuels with high sulfur content and to avoid problems related to same, the object of this invention is to control the speed of the gas exhauster placed on the kiln outlet and the fuel flow which is to be used in the process, to thus allow control of the oxygen amount in the chemical reaction zone/sintering of the clinkering kiln. This type of apparatus is inefficient for controlling the amount of SO3 in the resulting cement clinker and has delayed response time for correcting the operating conditions.
Another effort in the art to use petroleum coke is disclosed in the China patent number 1,180,674 issued to Wang Xinchang, et al., on May 6, 1998, entitled xe2x80x9cMethod to produce high quality cement using petroleum coke with a high sulfur contentxe2x80x9d. The method includes the steps of spheroidizing the petroleum coke mixture with high sulfur content, limestone, clay, iron dust, carbon and fluorite; calcining to obtain the clinker; mixing with gypsum; and pulverizing. The sulfur content of said coke could be from 2 to 6%, which can replace the carbon by 20-100%. The cement obtained up to more than 425R characteristics and high resistance. The method is based on mixing the raw materials with petroleum coke to later be granulated and burned in a vertical kiln. However, this creates problems with respect to the appropriate dosage of the mixture to carry out the decarbonation (calcination) of the calcinated mixture, which results in an inefficient reaction.
Further efforts are disclosed, for example, in U.S. Pat. No. 5,707,444 (Soren Hundebol), which relates to the amount of fuel used in the kiln. In adjusting the same in accordance with the calculation of the sulfur evaporation, this Hundebol patent focuses on the adjustment related specifically with the temperature control of the clinkering kiln. See also U.S. Pat. No. 6,050,813 (Joseph Doumet), which describes control of the cement clinker production by means of the operating parameters, such as by control of the amount of oxygen at the kiln inlet via the measurement of the SO3 content in the clinker at the cooler exit, the drawback being that the information on the SO3 obtained in the clinker to control the amount of oxygen fed is delayed and, thus, can not provide an adequate control on line for the production of a clinker, because the operating conditions do not correspond to those during the formation of the clinker being analyzed. See also U.S. Pat. No. 6,183,244 (Joseph Doumet), which relates to the production of clinker using fuel with high sulfur content in a wet rotating kiln.
The U.S. Pat. No. 5,698,027 entitled xe2x80x9cMethod and Plant for manufacturing mineralized Portland cement clinkerxe2x80x9d issued to Hans E. Borgholm, et al, on Dec. 16, 1997, describes the use of a mineralizer which has no detrimental effect on the flow properties of the raw meal, which is preferably added after the preheater for homogenization with the feedstock in the preheater or calciner. Said mineralizer can be, for example, a sulfur-containing component (such as a by-product in the desulfurization of the fuel gas) and/or a fluorine-containing compound. The mineralizer can be directly fed to the raw mixture, to the calciner or through hot air from the cooler towards the calciner. It does not mention the kiln temperature control or to its stabilization, to avoid sulfur evaporation provided as a by-product of desulfurization of the fuel gas, i.e., the decomposition of the sulfated compounds. This would generate problems in the system, such as the accumulation, blockage and the formation of rings due to said sulfur compounds.
It has also been considered to improve the burnability of the raw materials or raw meal for the purpose of reducing the sintering temperature (clinkering) and, thus, to avoid the decomposition temperature of the sulfated compounds to be achieved. Examples of methods and apparatus to burn granulated or pulverized products to manufacture cement clinker are described, for example in the U.S. Pat. No. 6,000,145 issued on Dec. 14, 1999; U.S. Pat. No. 4,496,396 issued on Jan. 29, 1985; U.S. Pat. No. 4,561,842 issued on Dec. 31, 1985; and U.S. Pat. No. 4,557,688 issued to Nielsen Peter B., on Dec. 10, 1985. However, all the above-mentioned patents are related to the preparation of raw materials as a conventional phase in the clinker manufacturing process, substituting the rotating kiln with stationery reactors, without taking into consideration the decomposition of the sulfur compounds. In a relevant form, the U.S. Pat. No. 5,800,610 issued to Ebbe S. Jons on Sep. 1, 1998, entitled xe2x80x9cMethod for manufacturing cement clinkerxe2x80x9d, describes to initially subject the raw meal to a preparatory processing, e.g., by comminution, homogenization and/or drying. Nevertheless, this method is carried out on fixed burning reactor, and there is no mention of the form in which said preparatory processing is carried out nor of the operating conditions.
According to the above, there is no document in the prior art that describes or suggests the reduction of the clinkering or sintering temperature by means of parameters to control of the raw mixture, in the operation of the kiln and the SO3 level at the kiln inlet. Thus, there is the need for a method for producing cement clinker which will allow a more economic and efficient use of fuels with a high sulfur content, such as petroleum coke, but that at the same reduces to the minimum the problems associated with blocking and incrustations due to the high concentration of SO2 and/or SO3 in the system.
Therefore, the object of the present invention is to provide a method to produce cement clinker with a simple operation and, however that is highly efficient to consume 100% of the petroleum coke.
It is a further object of the present invention is to produce a high quality cement clinker, which does not require the addition of additives to improve its final physical properties.
Yet, another object of the present invention is to provide a method to produce cement clinker that can be used in any conventional process and facility; thus avoiding the need for any added capital costs to accommodate for use of fuels with high sulfur content.
The present invention relates to a method for producing cement clinker in a cement plant cement clinker from a conventional feedstock raw mixture, which method comprises the steps of:
feeding the raw mixture to said plant;
preheating the raw mixture;
calcining the preheated raw mixture;
sintering of the calcinated raw mixture to produce cement clinker; and
cooling the cement clinker;
modifying the method to permit use of a fuel having a high sulfur content as a heat source for at least one of preheating, calcining, decarbonating and sintering of the raw mixture while preventing process-inhibiting build up of sulfur compound encrustations within said plant by:
preparing a modified raw mixture before the feeding step based on the Lime Saturation Factor (LSF), the Silica Module (SM) and the Alumina Module (AM) and the fineness of the raw mixture, wherein the LSF is up to 1, the SM is between 2 and 3.5, the AM is between 0 and 3, and the fineness is on the order of mesh size no. 200, which in combination are adequate to reduce the conventional temperature in the clinkering step by more or less 100xc2x0 C., from about 1500-1450xc2x0 C. down to about 1400-1350xc2x0 C., while maintaining essentially the same low percent of free CaO; and
controlling the residence time of the calcinated mixture, the clinkering temperature and the maintenance of oxidizing conditions at the kiln inlet at an excess of 4% to 5% during the clinkering step.
The fuller scope of the invention in its broader aspects and its detailed working is set forth below.